This invention relates to a method of shaping a thermoplastic sheet, whereby an article such as a canopy for the pick-up box of a pick-up truck, or the liner of a shower stall, may be formed. Such an article generally has two spaced apart and opposed sides, and a joining expanse extending transversely of these sides and together with the sides providing a generally U-shaped profile to the article.
The molding process known in the art for manufacturing parts such as pick-up truck canopies, utilizes a so-called "snap back" system. Describing the system, a sheet of material is first heated whereby it becomes soft and tends to droop of its own weight inwardly of the supported margins of the sheet. A vacuum box with a hollow interior is then brought up against the underside of the material, and the material caused further to sag into the interior of the vacuum box by the application of a vacuum. The mold that finally shapes the material is then inserted into the vacuum box, into the concave region of the deformed sheet. A vacuum is then drawn through the mold, which causes the sheet material to snap back and to cling to the periphery of the mold. The vacuum box is then removed and the formed sheet of material cooled and removed from the mold.
A problem resulting from the use of such a system is exemplified by the fact that the sheet material has a thickness after shaping which might be, for instance, 40% of the thickness of the original sheet. By using the vacuum box, an effort is made to have this reduction in thickness uniform throughout the sheet, but this is rarely achieved. At regions such as corners of the molded object, the thickness tends to be considerably less than the thickness of the sheet in other regions. As a consequence, the molded product has a non-uniform thickness throughout.
Plastic sheeting used in making product frequently may be a laminated type product, with an outer so-called rovel layer of expensive plastic which is the exterior of the formed product, and relatively thin, and a back-up layer of so-called ABS plastic which is cheaper and gives structural strength. When molding a product from a laminated sheet, problems arising because of unevenness of the sheet are magnified.
Additionally, and in making a canopy, such usually has a top and two sides. A product produced as just described tends to have a top and four sides, meaning that two of the opposed sides have to be trimmed away to produce the final canopy. The cut away material is recycled, but still represents an expense figure in making the canopy.
The method of the invention features pre-forming of a sheet of material from manipulation of a support for the sheet whereby the sheet is shaped to have two opposed side portions and a connecting expanse, similar to the shape of the final product. Associated with the pre-formed sheet is a mold which has mold surfaces forming a pair of sides and an interconnecting expanse. Pre-shaping is done with the pre-shaped sheet extending in close adjacency to the mold surfaces described. With pre-shaping and positioning of the mold, a vacuum may be applied to the mold to bring the sheet firmly against the mold surfaces and to form the final sheet. There is minimal uneven stretching of the sheet whereby thick and thin regions are produced. Products may be formed which are two-sided rather than four-sided, thus minimizing wastage of the plastic material. The formed plastic body having two sides, and without four sides, may be flexed slightly to produce freeing of so-called undercut regions, not easily performed with a four-sided molded body. Flexibility is provided in the types of body shapes which can be produced with the system of the invention.
A general object of the invention, therefore, is to provide a novel molding method which includes pre-shaping of a sheet supported on a support means, with pre-shaping performed by manipulating the support means to produce the shape desired.
Another object is to provide a method of molding a plastic sheet which makes possible the molding of bodies having only two sides joined by an expanse, rather than four sides.
A further object is to provide a molding method which minimizes the production of thick and thin spots in the molded product.
A further object is to provide a molding method which permits release easily of the molded product even though such may have so-called undercut regions which would make removal of such a product from a mold nearly impossible with a conventional snap back technique.
According to one embodiment of the invention, the sheet is preformed, and positioned so that its opposed side portions and connecting expanse extend in close adjacency to the external surfaces of a male mold. With a vacuum applied, the sheet is drawn against the mold surface to form an article such as a canopy where external surfaces have original sheet smoothness without taking the imperfections of the mold.
In another embodiment, the sheet is preformed and positioned so that its opposed side expanses and connecting expanse extend in close adjacency to the internal surfaces of a female mold. With a vacuum applied, the sheet is drawn against the internal mold surfaces to form an article such as a shower stall liner where internal surfaces have original sheet smoothness without taking the imperfections of the mold.